Low-Level Design Interview Prep
Test & Track Your Readiness

The four pillars — encapsulation, abstraction, inheritance, polymorphism — plus Java-specific rules every OOD interview tests: equals/hashCode contract, final semantics, cohesion vs coupling

The named principles beyond SOLID — DRY, KISS, YAGNI, Tell Don't Ask, CQS, Composition over Inheritance, Program to Interfaces — and exactly when violating each one is justified

Five design rules that prevent class rot — how each principle is violated in real codebases and the minimal refactor that fixes it

Control how objects are born — Singleton, Factory, Abstract Factory, Builder, Prototype — and the initialization complexity problems each one solves

Compose objects into larger structures — Adapter, Decorator, Proxy, Facade, Composite, Bridge, Flyweight — without rewriting existing code

Manage algorithms and object interactions at runtime — Strategy, Observer, Command, State, Mediator, Chain of Responsibility, Template Method

Read and draw class diagrams that communicate intent — associations, aggregation, composition, dependency, realization, and multiplicity notation

Wire abstractions to implementations without hardcoding — constructor vs setter vs field injection, IoC containers, service locator anti-pattern, and how DI makes designs testable and swappable

The repeatable framework for 60-minute coding interviews — scope requirements, discover entities, assign responsibilities, then write clean extensible code

Design exception hierarchies that communicate failures clearly — checked vs unchecked, custom exception trees, Optional vs throw, fail-fast contracts, and why catching Exception is an OOD smell

Design thread-safe objects in Java — synchronized, volatile, locks, immutability, ReadWriteLock, Semaphore, and concurrency patterns like Producer-Consumer and Thread Pool

Eliminate code smells before they become design debt — naming, function size, Law of Demeter, God class, Feature Envy, Primitive Obsession, and the refactoring moves that fix each one